Downhole Apparatus Using Induction Motors with Magnetic Fluid in Rotor-Stator Gap

ABSTRACT

In one aspect, an apparatus for use in a wellbore is disclosed that in one non-limiting embodiment includes an AC motor having a rotor and a stator with a gap between the rotor and the stator and a magnetic fluid in the gap that contains an electrically nonconductive fluid and magnetic nanoparticles.

BACKGROUND

1. Field of the Disclosure

This disclosure relates generally to AC induction motors and to downhole apparatus that utilizes such motors, wherein a magnetic fluid is utilized between the rotor and stator to increase the efficiency of such motors.

2. Background of the Art

Wells (also referred to as “wellbores” or “boreholes”) are drilled in subsurface formations for the production of hydrocarbons (oil and gas). Wellbores often extend to depths of more than 5000 meters (over 15,000 ft.). Many such wellbores are deviated or horizontal. After a wellbore is formed, a casing is typically installed in the wellbore, which is perforated at hydrocarbon-bearing formation zones to allow the hydrocarbons to flow from the formation into the casing. A production string is typically installed inside the casing. The production string includes a variety of flow control devices and a production tubular that extends from the surface to each of the perforated zones. Some wellbores are not cased and in such cases the production string is installed in the open hole. Often, the pressure in the hydrocarbon-bearing subsurface formations is not sufficient to cause the hydrocarbons to flow from the formation to the surface via the production tubing. In such cases, one or more electrical submersible pumps (ESPs) are often deployed in production string to lift the hydrocarbons to the surface.

An ESP includes a pump driven by an AC induction motor. The rotor and stator of an AC Induction motor are separated by a gap that creates a magnetic field disconnect between the rotor and the stator, which generates a reluctance load within the motor and causes the stator to pull additional current. Additional current pulled by the stator makes the motor inefficient and also generates heat that increases the already high temperature of the motor in the wellbore, which temperature can exceed 300° F. In an AC induction motor, the highest reluctance and thus greatest loss of the magnetic field between the stator and the rotor is due to the gap between the rotor and stator because the medium in the gap (air in most AC induction motors with dielectric oil in most ESP AC induction motors) has low magnetic permeability. Therefore, increasing the magnetic permeability (i.e. reducing the reluctance) of the medium in the gap can improve the overall efficiency of an AC induction motor, reduce the heat generated by the motor and increase the overall efficiency and the operating life of the motor.

The disclosure herein provides apparatus and methods that in general improve the overall performance of AC induction motors, and particularly motors utilized in ESP pumps for downhole applications.

SUMMARY

In one aspect, an apparatus for use in a wellbore is disclosed that in one non-limiting embodiment includes an electric motor with a gap between a rotor and a stator and a magnetic fluid in the gap that contains an electrically nonconductive fluid and magnetic nanoparticles that increase the magnetic permeability of the gap.

In another aspect, a method of producing a fluid from a wellbore is disclosed that in one non-limiting embodiment may include: deploying a string in the wellbore that includes a pump driven by an electric motor that includes a magnetic fluid in a gap between a stator and rotor of the electric motor; and operating the pump with the electric motor to produce the fluid from the wellbore.

Examples of the more important features of the apparatus and methods of the disclosure have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features that will be described hereinafter and which will form the subject of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding of the apparatus and methods disclosed herein, reference should be made to the accompanying drawings and the detailed description thereof, wherein like elements have generally been given like numerals and wherein:

FIG. 1 is a schematic diagram of an exemplary production wellbore with an ESP deployed therein, which ESP is made according to one non-limiting embodiment of the disclosure;

FIG. 2 shows an exemplary motor of an ESP that includes an electrically nonconductive magnetic fluid according to one non-limiting embodiment of the disclosure;

FIG. 3 shows a cut-view of the motor section “A” of the induction motor of FIG. 2 showing a magnetic fluid in the gap between the stator and rotor;

FIG. 4 shows a cut-view of the motor section “B” shown in FIG. 2; and

FIG. 5 shows a non-limiting embodiment of a heat-exchange fluid reservoir and a device that mixes magnetic nanoparticles with a base fluid in the heat-exchange reservoir for circulating the magnetic fluid in the motor of FIG. 2.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows an exemplary wellbore system 100 that includes a wellbore 110 drilled from the surface 104 through the earth formation 102. The wellbore 110 is shown formed through a production zone 120 that contains hydrocarbons (oil and/or gas) therein. The fluid 150 in the production zone 120 is referred to as the “formation fluid” and typically contains hydrocarbons and water. System 100 includes a string that includes an electrical submersible pump (ESP) 160 that contains a pump 184 driven by an electric motor 180, typically an AC induction motor). Seals 186 separate the motor 180 and the pump 184. The formation fluid 150 enters the wellbore 110 from the production zone 120 via perforations 116 and control equipment 130, such as sand screens, valves, etc. known in the art. The formation fluid 150 enters the pump 184 as shown by arrows 162. The production zone 120 is shown isolated from the wellbore 110 above and below perforations 116 by packers 122 a and 122 b. The wellbore section between the packers 122 a and 122 b is filled with the formation fluid 150. The ESP 160 is shown deployed on a production tubing 140 for lifting the formation fluid 150 from the production zone 120 to the surface 104 via the production tubing 140. The fluid level in the wellbore 110 is maintained a certain level above the ESP 160 to provide a fluid head to the ESP. Power to the ESP 160 is supplied from a power source 162 at the surface and a controller 164 may be utilized to control the operations of the ESP 160. A fluid processing unit 170 at the surface 104 processes the formation fluid 150 received at the surface 104. Various sensors 188 may be utilized for determining information about one or more parameters relating to the ESP 160, including, but not limited to, temperature, pressure and vibration.

FIG. 2 shows an exemplary motor 180 of an ESP that includes a magnetic fluid 270 in the gap between the rotor and the stator of the motor 180, according to one non-limiting embodiment of the disclosure. Referring to FIGS. 1 and 2, the motor 180 includes a housing 210, a base 212 and an upper threaded end 214 for connection to the seals 186. The motor 180 includes a stator (laminations) 220 and a rotor 230 that rotates a shaft 240. Bearings 250 support the rotor 230 and the shaft 240. The motor 180 further includes a reservoir or chamber 260 that includes the magnetic fluid 270. In one non-limiting embodiment, the magnetic fluid 270 may include any electrically nonconductive fluid 272, such as oil, for lubricating the various components of the motor 230 and a selected amount (by weight or volume) of magnetic nanoparticles 280. The magnetic nanoparticles 280 increase the magnetic permeability of the fluid 272 in the gap and thus reduce the magnetic reluctance of the gap, which reduces the reluctance of the gap between the stator 220 and the rotor 230. In operations, the rotor 230 rotates the shaft 240 at a relatively high rotational speed, which speed may exceed 3000 rpm. The magnetic fluid 270 moves up the shaft 240 and passes around the bearings 250 and in the gap between the stator 220 and the rotor 230, which in aspects may provide relatively high magnetic permeability in the gap compared to the base fluid 272.

FIG. 3 shows a cut-view of the motor section “A” shown in FIG. 2. View 300 shows stator 220 with windings 320 and rotor 230 with shaped rotor bars 330. The gap 380 between the stator 220 and the rotor 230 is filled with the magnetic fluid 270. The magnetic fluid 270 circulates in the gap 380 due to the movement of the fluid 270 from the reservoir (260, FIG. 2) to the various components of the motor 180, including the gap 380. Typically, commercially available AC induction motors do not have the various components described herein. Such motors include a stator and a motor separated by ab air gap. In such motors, the gap may filled with a sealed magnetic fluid that includes a base fluid, such as oil or another suitable electrically non-conductive fluid, and magnetic nanoparticles to increase the magnetic permeability of the gap 380.

FIG. 4 shows a cut-view 400 of motor section “B” shown in FIG. 2. View 400 shows the housing 210 containing stator laminations 220, rotor 230 with end rings 332, and shaft 240 supported by bearings 250 a. The magnetic fluid 270 moves along the path 445 as shown by arrow 370. The magnetic fluid 270 circulates around the bearing 250 a via fluid passages 420 as shown by arrow 475 and returns to the reservoir 260 (FIG. 1) via fluid passages 480 as shown by arrow 485 respectively. Typically, there is more than one set of bearings. The magnetic fluid 270 also circulates around bearings that are uphole of the bearing 250 a and such fluid returns to the reservoir 260 via a passage, such as passage 488.

FIG. 5 shows a non-limiting embodiment of a reservoir that includes or has associated therewith a device that mixes the nanoparticles 280 with the base fluid 272 in the reservoir. In one aspect, the shaft 240 may be extended, as shown by extension 510 and a mixer 520 attached to the shaft extension 510. In one non-limiting embodiment, the mixer 520 may include any type of mixing mechanism, including, but not limited to, propellers and fins that continuously churn the magnetic fluid 270 in the reservoir 260.

In aspects, the magnetic fluid (270, FIGS. 2-4) may comprise any or more suitable electrically nonconductive fluids and selected amounts of one or more suitable magnetic nanoparticles. For the purpose of this disclosure, the term “nano” includes nano-meter size particles and/or micro-meter size particles. In one non-limiting embodiment, the size of the magnetic nano particles may be less than about 12 nm and in another non-limiting embodiment the size of the magnetic nanoparticles may be less than 100 nm. In other aspects, the nanoparticles may be of the form AB₂O₄, in one embodiment, A may be iron, cobalt, manganese, zinc, nickel and combinations thereof and B is iron. In one aspect the size of the AB₂O₄ particles is less than 100 nm. In another non-limiting embodiment, the magnetic nanoparticles include Fe₃O₄. In one aspect, the size of the Fe₃O₄ particles may be less than 12 nm. In other embodiments, magnetic nanoparticles may include electrically and magnetically conductive elements with an electrically non-conductive coating. In aspects, the conductive elements may include, but are not limited to, iron, cobalt, nickel, and their alloys. In each embodiment, the magnetic nanoparticles are electrically nonconductive and magnetically permeable. In aspects, the magnetic nanoparticles are suspended in the base fluid during circulation of the fluid through the gap. In another aspect, the amount of the magnetic nanoparticles in the magnetic fluid is selected to maintain an operating viscosity of the magnetic fluid in the gap between a desired range. In one aspect, the viscosity range of the magnetic fluid at the operating temperature is between 3 and 6 cP.

Referring to FIGS. 1-4, in general, reducing the reluctance of the gap 380 by providing higher magnetic permeability gap, such as by filling the gap with a magnetic fluid, increases the power factor of the motor because of the reduced inductive loads within the motor. The reduced inductive load causes the motor to draw (consume) less current for the same amount of motor break horsepower (BHP) as of a motor with a lower magnetic permeability gap, such as motor with an air gap or filled with an oil. Thus, utilizing a magnetic fluid in the gap increases the efficiency of the motor. Additionally, the reduced amount of the current used by the motor results in lower heat generated by the stator windings and further improves the overall efficiency of the motor. In one aspect, providing a magnetic fluid in the gap between the stator and the rotor of an ESP may increase the power factor of the ESP motor by about 15 percent and decrease the internal temperature in the motor by about 10° F. Additionally, reduced use of the current increases the reliability of the motor because the internal temperature of the motor is reduced. Additionally, simulations demonstrate that suspended magnetic nanoparticles within the base fluid 272 in the gap 380 follow the rotating magnetic field flux lines in the motor, accelerating the fluid 270 around the gap 380 until it reaches a steady state velocity. Such a phenomenon can reduce the friction losses within the motor as shearing losses are reduced. Thus, incorporating electrically nonconductive magnetic particles in a lubricating fluid in the gap between the rotor and the stator of an AC induction motor increases the overall performance of the AC induction motor. In motors that typically include an air gap, use of a magnetic fluid, such as mixture of oil and magnetic nanoparticles, may increase the efficiency of the motor to an extent that offsets the reduction in the efficiency due to the friction losses created by the magnetic fluid in the gap when the rotor rotates in the stator.

The foregoing disclosure is directed to certain exemplary embodiments and methods. Various modifications will be apparent to those skilled in the art. It is intended that all such modifications within the scope of the appended claims be embraced by the foregoing disclosure. The words “comprising” and “comprises” as used in the claims are to be interpreted to mean “including but not limited to”. Also, the abstract is not to be used to limit the scope of the claims. 

1. An apparatus for use in a wellbore, comprising: a motor having a rotor and a stator with a gap between the rotor and the stator; and a magnetic fluid in the gap that contains an electrically nonconductive fluid and magnetic nanoparticles that increase magnetic permeability of the electrically nonconductive fluid.
 2. The apparatus of claim 1 further comprising a pump driven by the motor.
 3. The apparatus of claim 1, wherein the magnetic nanoparticles are electrically nonconductive.
 4. The apparatus of claim 1, wherein the magnetic nanoparticles comprise a composition of AB₂O₄, wherein A is chosen from a group consisting of iron, manganese, cobalt, zinc, nickel and a combination thereof and B is iron.
 5. The apparatus of claim 1, wherein the magnetic nanoparticles include a core having an electrically-conductive magnetic material and a shell made from an electrically nonconductive material.
 6. The apparatus of claim 5, wherein the core includes a material selected from a group consisting of: a metal; nickel; iron; cobalt; and a combination thereof.
 7. The apparatus of claim 1, wherein the magnetic nanoparticles are suspended or substantially suspended in the electrically nonconductive fluid.
 8. The apparatus of claim 1, wherein size of the magnetic nanoparticles is selected from a group consisting of: less than 12 nm; and between 12 nm and 100 nm.
 9. The apparatus of claim 1, wherein amount of the magnetic nanoparticles in the magnetic fluid is selected to maintain an operating viscosity of the fluid in the gap between a desired range.
 10. The apparatus of claim 1, wherein the magnetic nanoparticles cause the magnetic fluid in the gap to move with magnetic field lines between the stator and the rotor to reduce friction loss caused by the electrically nonconductive fluid.
 11. The apparatus of claim 1 further comprising: a reservoir that contains the magnetic fluid; and a fluid circulation device that circulates the magnetic fluid in the motor.
 12. The apparatus of claim 11, wherein the circulation mechanism includes fins that cause the magnetic nanoparticles to mix with the electrically nonconductive fluid in the reservoir.
 13. A production system comprising: a production string in a wellbore including a tubing; an electrical submersible pump that supplies a fluid from the wellbore to the tubing, wherein the electrical submersible pump includes: a pump; a motor having a gap between a stator and a rotor; and a fluid in the gap that contains an electrically nonconductive fluid and magnetic nanoparticles that increase magnetic permeability of the nonmagnetic fluid.
 14. The apparatus of claim 13, wherein the magnetic nanoparticles are selected from a group consisting of: a material having composition of AB₂O₄, wherein A is selected from a group consisting of iron, manganese, zinc, cobalt, nickel and a combination thereof; and particles having an electrically-conductive core and an electrically nonconductive shell.
 15. A method of making an apparatus, comprising: providing a rotor inside a stator, with a gap between the rotor and the stator; and filling the gap with a magnetic fluid.
 16. The method of claim 15, wherein the magnetic fluid includes an electrically nonconductive fluid and magnetic nanoparticles.
 17. A method of producing a fluid from a wellbore, the method comprising: deploying a string in the wellbore, the string including a pump driven by an motor, wherein the motor includes a rotor and a stator with a gap between the rotor and the stator and a magnetic fluid in the gap; and operating the pump with the motor to produce the fluid from the wellbore.
 18. The method of claim 17, wherein the motor includes a fluid reservoir configured to circulate the magnetic fluid through the gap.
 19. The method of claim 17, wherein the magnetic nanoparticles are selected from a group consisting of: a material having composition of AB₂O₄, and particles having an electrically and magnetically-conductive core and an electrically nonconductive outer surface.
 20. The method of claim 17, wherein size of the magnetic nanoparticles is selected from a group consisting of: less than 12 nm; and between 12 nm and 100 nm. 